To rapidly develop new and better therapies to combat drug resistant malaria, we must fully elucidate the molecular mechanisms of antimalarial action and antimalarial drug resistance. In addition, more rapid and convenient biochemical and physiological assays for various antimalarial drug resistance phenotypes are desperately needed. Both of these issues are effectively addressed via more detailed, single-cell level analysis of malarial parasite physiology and biophysics. We thus propose a detailed analysis of transport, pH, and membrane potential phenomena in living intraerythrocytic malarial parasites using modern subcellular imaging and single - cell photometry methods. Specifically, we will: 1) Test putative relationships between cytosolic and vacuolar pH (pHcyt, pHvac) and membrane potentials of living intraerythrocytic P. falciparum and resistance to antimalarial drugs using single-cell photometry of cells under constant perfusion and laser confocal microscopy. 2) Elucidate ion dependencies, inhibitor sensitivities, and kinetic characteristics of pHcyt, pHvac, and membrane potential regulation. 3) Test conclusions from 1,2 using progeny of genetic cross between resistant and sensitive parasites. 4) Define effects of antimalarial drugs and reversal agents on biophysical parameters. 5) Compare and contrast measurement with laboratory strains vs. field isolates of P. falciparum and P. vivax. Along with defining the ion - transport physiology of drug resistant parasites, these experiments will be performed with an eye towards development of principles that will aid in the rapid, "field compatible", diagnosis of drug resistant malaria.